Demand or VVI pacers are known from U.S. Pat. No. 3.057,356 to Wilson Greatbatch. Such pacers have a sense amplifier for detecting cardiac depolarizations in the ventricle of the heart. Upon the occurrence of a ventricular sense event, the pacer recycles and restarts the pacer's V-to-V timer circuitry. In the absence of a ventricular sense event, the pacer timer times out, generating a stimulation pulse which is applied to the heart.
In early pacers of the type taught by Greatbatch, the output pulse amplitude was on the order of 10 volts. More modern pacers, as typified by the teachings of U.S. Pat. No. 4,340,062 to McDonald, et al provide an output pulse amplitude which is programmable from approximately 1.5 volts to 5 volts. The sense amplifier circuits and timing circuits within a modern pacer operates from a supply voltage on the order of 2.8 volts which is the nominal open circuit potential of lithium chemistry batteries or cells. The programmable output voltages are derived from voltage divider and multiplier circuits associated with the output circuitry of the pacer.
The preceding pacers have only one sense amplifier coupled to the ventricular chamber of the heart, however, improved therapy may be achieved by pacers which sense and pace in both chambers of the heart. U.S. Pat. No. 4,312,355 to Funke is an example of a DDD or atrial and ventricular synchronized pacer which senses and paces in the atrium and the ventricle.
When dual chamber sensing is provided in a pacer with multiprogrammable output amplifiers reliable electrogram sensing becomes more difficult to achieve.
An output stimulus provided to one chamber of the heart is propagated throughout the heart and can induce a large potential difference on the sensing/pacing lead in the unstimulated chamber. Although the induced differential signal may be small, the common mode potential may be quite large and can on occasion exceed the nominal supply voltage of the battery.
Traditional sense amplifier circuitry suffers from reduced common mode rejection as the common mode voltage approaches the power supply rails. This can lead to false sensing by generating a false positive detection of the common mode signal or false negative detection through failure to trigger on a cardiac induced differential signal superimposed on a large common mode voltage. This problem is addressed by the present invention.